My problem has cropped up suddenly - 'sharing' a MOBI file with my Kindle via email is failing. I can send the same file via the Gmail web client and that works fine.

There aren't too many variables here - Kindle's email address has been verified. Here's the log:

Log:
Find mail exchanger for kindle.com
Traceback (most recent call last):
File "site-packages\calibre\utils\smtp.py", line 120, in sendmail_direct
File "site-packages\calibre\utils\smtplib.py", line 334, in connect
File "site-packages\calibre\utils\smtplib.py", line 309, in _get_socket
File "socket.py", line 575, in create_connection
error: [Errno 10060] A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond